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Imagine three input keys, a, b and c. These can be seen as
columns in a spreadsheet. The first row of the spreadsheet would
therefore be a[0],b[0],c[0]. Lexical sorting orders the different
rows by first sorting on the on first column (key), then the second,
and so forth. At each step, the previous ordering is preserved
when equal keys are encountered.

Parameters:

keys : (k,N) array or tuple containing k (N,)-shaped sequences

The k different “columns” to be sorted. The last column is the
primary sort column.